The shift reaction is exothermic and low temperatures favour CO-conversion. Thus, the lower the temperature, the more a synthesis gas will be shifted towards CO2+H2 provided that the gas is contacted with a sufficiently active shift catalyst. Due to the exothermicity of the shift reaction, however, the synthesis gas is usually equilibrated in at least two steps, the first step being operated at a higher temperature than the second step. It is thus common practice to distinguish between carrying out the shift reaction at low temperature (typically 180–300° C., low temperature shift) and at high temperature (typically 300–500° C., high temperature shift).
The current catalyst of choice for high temperature shift is iron oxide, usually in admixture with chromium oxide. This catalyst, however, has the disadvantage that it forms methane if the synthesis gas has too low steam content compared to the content of carbon—in other words, if the oxygen/carbon ratio is below a certain critical value, which is a function of temperature. At temperatures above 500° C., some methane formation is always observed. Furthermore, the catalyst deteriorates very fast at 500° C. and above.
The catalyst materials of the present invention are comprised by a microscopic mixture of manganese oxide and zirconium oxide (Mn—Zr oxide) optionally with other oxidic promoters and optionally with metal promoters.
The catalyst materials of the present invention have the advantage of having very high stability and extremely high selectivity for the water gas shift reaction (i.e. no hydrocarbon formation) and may therefore replace or supplement the traditional iron-based catalysts. When the materials of the present invention are promoted with copper, a significant boosting of the activity is achieved. Promotion of the Mn—Zr oxides with metallic silver has a similar though less pronounced effect of boosting the activity.
Another advantage of the catalysts of the present invention compared to traditional high-temperature water gas shift catalysts is that these materials have superior adhesion properties towards other ceramic materials as well as towards metals. The catalysts of the present invention are therefore highly suitable for the manufacture of catalysed hardware, which may find use in stationary as well as automotive units in which a water gas shift active catalyst is desired.
It is well known that manganese oxide and zirconium oxide separately have some activity for catalysing the water gas shift reaction. It is highly surprising, however, that there is a strong synergistic effect between these oxides. Thus, a microscopic mixture of manganese oxide and zirconium oxide has a much higher catalytic activity than any of the pure oxides, especially after a short time on stream. As is demonstrated in the examples of the present invention under comparable conditions at 450° C. pure manganese oxide has a conversion of 41–42%, pure zirconium oxide has a conversion of 9–11%, while mixed manganese-zirconium oxide catalyst has a conversion of 58–60%. In all cases the equilibrium conversion amounts to 65% under the conditions of operation.
The synergistic effect of manganese oxide and zirconium oxide is particularly surprising in view of the fact that similarly prepared Mg/Zr and Mn/Ti oxides have very low activity. In fact, the Mn/Ti oxide has even lower activity (8–16% conversion under the same conditions as in the above examples) than pure manganese oxide. The Mg/Zr oxide has slightly higher activity (14–17% conversion under similar conditions as in the above examples) than pure zirconium oxide, but this is due to the fact that magnesium oxide itself is a more active catalyst for the shift reaction than zirconium oxide.
Furthermore, the mixed manganese-zirconium oxide catalysts have the surprising advantage of being extremely selective. As is demonstrated in the examples of the present invention, even exposure of these materials to dry synthesis gas does not result in any appreciable formation of methane. At a GHSV of 10000 Nl/g/h only 100 ppm methane was formed (0.01%) at 500° C. and 1000 ppm methane (0.1%) at 600° C. In fact, the selectivity may prove to be even higher, since even microscopic impurities of a number of transition metals under these conditions would result in methane formation.